1. Field of the Invention
This invention relates to a method of making a magnetic oxide film, and more particularly to a method of making a magnetic oxide film of excellent magnetic properties which is suitable for use in a recording medium of magnetic disks.
2. Description of the Prior Art
Conventional methods for the manufacture of a magnetic oxide film for magnetic disks include, for example, a particulate coating method, the method of making a continuous thin film by heating a film of .alpha.-Fe.sub.2 O.sub.3 from a solution of ferric chloride and reducing it to magnetite, a method in which a film of iron or non-magnetic .alpha.-Fe.sub.2 O.sub.3 formed by sputtering or the like is rendered into a ferromagnetic oxide, and the method of making alternating layers of magnetic iron and non-magnetic iron oxide (hematite .alpha.-Fe.sub.2 O.sub.3) on a substrate under a controlled sputtering atmosphere.
In the particulate coating method, a binding material is employed to provide for enhanced adhesiveness of oxide fine particles to the substrate and tightened binding of the fine particles to one another. This lowers the density of the magnetic oxide contained in the film ultimately obtained, and hence is not suitable for recording with very high storage density. Further, from the view-point of coating techniques, it is very difficult to form the film to a thickness of the order of 10.sup.3 A.
With the abovesaid method of making a continuous thin film by heating a film of .alpha.-Fe.sub.2 O.sub.3 from a solution of ferric chloride and reducing it to magnetite (refer to U.S. Pat. No. 3,620,841), the thickness of the film coated at one time is small, so that a coating and drying process must be carried out a plurality of times. Hence, this method is low in working efficiency.
With the abovesaid method of forming a magnetic oxide film of Fe.sub.3 O.sub.4 from a film of Fe or .alpha.-Fe.sub.2 O.sub.3 formed by sputtering or the like (refer to U.S. Pat. No. 3,795,542), it is necessary to oxidize Fe at 300.degree. to 450.degree. C to .alpha.-Fe.sub.2 O.sub.3 and, further, it is then necessary to reduce the .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4 in a controlled reducing atmosphere at 300.degree. to 350.degree. C. These operations involve the use of relatively large apparatus and, in addition, require several treatment steps. Further, since the treating temperature is above 300.degree. C, it is difficult to use an inexpensive substrate such as aluminum alloy.
The abovesaid method of making alternating layers of magnetic iron and non-magnetic iron oxide on a substrate under a controlled sputtering atmosphere has the disadvantages requiring sophisticated high-class apparatus and a high degree of control must be exercised, for example, apparatus for controlling the oxygen pressure by repeatedly changing the blending ratio of a rare gas and oxygen in the sputtering process and a control of the absolute values of the thicknesses of iron and iron oxide for maintaining the magnetic properties of the film constant are required.